1. Field of the Invention
The field of art to which this invention pertains is aromatic polyamide fibers and, more particularly, it is directed to processes for dyeing such fibers.
Specifically, such invention is a process for dyeing a fiber structure or tow of crystalline poly(meta-phenylene isophthalamide) fibers or filaments with a water-insoluble dye padded onto the filaments. The tow is heated with steam at a temperature below the glass transition temperature of the poly-(m-phenylene isophthalamide) but above the dye-activation temperature of the dye to activate the dye material on the surface of the filaments of the tow, and, thereafter, it is heated with steam at a temperature above the glass transition temperature of the poly(m-phenylene isophthalamide) but no more than about 165.degree. C. to diffuse substantially all of the dye material into the filaments of the tow.
Preferably the tow is heated to a maximum temperature of from about 150.degree. to 165.degree. C.
By following the process of this invention crystalline poly(meta-phenylene isophthalamide) filaments may be efficiently dyed in a short period of time with water-insoluble dyes, without the use of carriers or swelling agents, at relatively low temperatures, e.g., below 165.degree. C., without degradation of the dye material.
2. Description of the Related Art
Aromatic polyamide fibers are well known to the art. They have high tensile strength, are flame and heat resistant, possess good flex life, and have high melting points which make them particularly suited to be formed into fabrics usable as protective clothing, and for many other uses.
It further is known that while such aromatic polyamide fibers possess many desired properties as manufactured they also require, for given uses, that various steps be taken to improve a property or properties of the fibers to meet a specific end use. As an example, various additives such as dyes, ultraviolet light screeners, flame retardants, antistatic agents or water repellents, may be incorporated into the fibers during basic manufacture or in subsequent processing steps to improve their performance levels.
This invention is specifically directed to aromatic polyamide fibers of a poly(meta-phenylene isophthalamide) polymer, hereinafter referred to as "MPD-I fibers". Such fibers, which are described in greater detail in U.S. Pat. No. 3,287,324 to Sweeny, for example, possess many useful properties. It is well known to the art, however, that these fibers are difficult to dye.
Various techniques have evolved to solve this dyeing problem. Typical solutions, for example, are more fully described in copending applications Ser. No. 910,941, filed Sept. 26, 1986, and Ser. No. 055,394, filed May 29, 1987, which applications are incorporated herein by reference.
The inventions of these copending applications solve various problems found in the prior art by surprisingly finding that by heating as-spun, never-dried, water-swollen, or still moist to the touch, MPD-I fibers with steam, heated within certain temperature ranges, it is possible to effectively dye the fibers. Specifically, it has been found that such fibers may be dyed, using a water-soluble dye, by heating the fibers with steam heated at a temperature from about 110.degree. C. to 140.degree.C. for a time sufficient to diffuse the dye into the open pores of the fibers. A water-insoluble material, such as an ultraviolet light screen or a disperse dye, may also be mixed with the water-soluble dye, and driven into the fibers by heating with steam at sublimation temperatures from about 110.degree. C. to 150.degree. C.
It further has been found that these fibers may be subsequently heated, again with steam, at a temperature of about 165.degree. C. to collapse the fibers and lock the dye in place. This latter step will also crystallize the fibers and stabilize them against progressive laundry shrinkage.
Another solution to this dyeing problem is shown in British Pat. No. 1,438,067 to Moulds and Vance which teaches imbibing a polyoxyethylene laurate impregnant into never-dried MPD-I fibers, prior to dyeing. The impregnant serves as a "structure prop" which prevents collapse of the water-swollen fibers on drying. The dried fibers may subsequently be readily tinted in an aqueous dye bath while corresponding fibers dried without the impregnant may be tinted only under much more vigorous conditions, including necessarily the use of dye carriers, such as acetophenone.
This invention essentially involves a process for dyeing MPD-I fibers, in crystalline form. These fibers are particularly difficult to dye.
Again, various processes have evolved for producing crystalline MPD-I fibers in various colors. In one such process the dye is incorporated into the spinning solution prior to extrusion of the fibers through orifices in a spinneret, in a basic procedure well known to the art. The fibers are then crystallized.
In this process, the fibers are exposed to very high temperatures during crystallization and the spun-in dyes must be stable at the high temperatures involved. Only a few dyes, mainly certain acid dyes, are sufficiently stable for use as spun-in dyes.
The limitations on the type of dye which may be used to dye MPD-I fibers have also been addressed by the art. For Example, U.S. Pat. Nos. 3,558,267 to Langenfeld and 4,710,200 to Cates et al. disclose that almost any conventional dyestuff can be used to dye MPD-I fibers, including crystalline MPD-I fibers, by making a solution of the dye in a liquid which is a solvent or strong swelling agent for the fiber, or in concentrated aqueous solution of the liquid, and heating the fiber in the resulting solution. The problem with this approach to coloring crystalline MPD-I fibers is that the fiber properties are usually adversely affected by the solvent or swelling agent. Also, recovery of the liquid remaining after dyeing or disposing of it in a non-polluting manner is a problem.
Lastly, in the art, the prime conventional method that has evolved for dyeing uncolored, crystalline MPD-I fibers has been to dye them with cationic dyes (water-soluble dyes which are also called "basic" dyes) in a pressure vessel using an aqueous dyebath containing several percent of a swelling agent (usually called a "carrier") at a temperature of about 121.degree. C. This approach has had limitations in that cationic dyes are the only ones suitable for use in dyeing MPD-I fibers from a substantially aqueous solution, and several hours are usually required to achieve the depth of color desired. Further, the disposal of the residual dyebath containing swelling agent in a non-polluting manner is a problem. Also, this method is more suitable for dyeing fabric than for dyeing tow.
Accordingly, a process has long been sought for dyeing crystalline MPD-I tow using disperse dyes or other dyes which cannot currently be applied to MPD-I fibers from substantially aqueous dyebaths to obtain a wide range of colors and retain good fiber properties. It has been especially desired to achieve a process for applying such dyes at relatively low temperature, e.g., 165.degree. C. or less, since many otherwise desirable dyes are unstable at higher temperatures. And, it further has been desired to be able to dye crystalline MPD-I tow continuously within a relatively short time, e.g., 30 minutes or less.
This invention solves these and other problems found in the prior art by surprisingly finding that by heating crystalline MPD-I fibers with steam, heated within certain temperature ranges, it is possible effectively to dye the fibers. Specifically, it has been found that such crystalline fibers may be dyed with a water-insoluble dye by heating the tow with steam at a temperature below the glass transition temperature of poly(m-phenylene isophthalamide), e.g., the MPD-I fibers, but above the dye-activation temperature of the dye to activate the dye material padded onto the surface of the fibers or filaments of the tow, and thereafter heating the tow with steam at a temperature above the glass transition temperature of the MPD-I fibers but no more than about 165.degree. C. to diffuse substantially all of the dye material into the filaments of the tow. Preferably the tow is heated to a maximum temperature of from about 150.degree. to 165.degree. C. and is dyed in a very short period of time, (e.g., less than 30 minutes) with no residual disposal problem. In so doing the process of this invention provides the art with an effective, improved means of dyeing crystalline MPD-I fibers with a large variety of dyestuff.